docs: def _checkMode(self, ax_args):
          """Raise an exception if the mode in the attribute exchange
          arguments does not match what is expected for this class.
  
          @raises NotAXMessage: When there is no mode value in ax_args at all.
  
          @raises AXError: When mode does not match.
          """
          mode = ax_args.get('mode')
          if mode != self.mode:
              if not mode:
                  raise NotAXMessage()
              else:
                  raise AXError(
                      'Expected mode %r; got %r' % (self.mode, mode))